Method and fingerprint sensing system for forming a fingerprint representation

ABSTRACT

The present invention generally relates to a method for forming a fingerprint using a fingerprint sensing system, and specifically to the possibility of allowing the formation of a fingerprint when only separated portions of the fingerprint is available. The invention also relates to the corresponding fingerprint sensing system and to a computer program product.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Swedish Patent Application No.1550689-2 filed May 28, 2015. The disclosure of the above application isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention generally relates to a method for forming afingerprint using a fingerprint sensing system, and specifically to thepossibility of allowing the formation of a fingerprint when onlyseparated portions of the fingerprint is available. The invention alsorelates to the corresponding fingerprint sensing system and to acomputer program product.

BACKGROUND OF THE INVENTION

Various types of biometric systems are used more and more in order toprovide for increased security for accessing an electronic device,thereby providing an enhanced user convenience. In particularfingerprint sensors have been successfully integrated in such devices,for example, thanks to their small form factor, high performance anduser acceptance. Among the various available fingerprint sensingprinciples (such as capacitive, optical, thermal etc.), capacitivesensing is most commonly used, in particular in applications where sizeand power consumption are important issues.

All capacitive fingerprint sensors provide a measure indicative of thecapacitance between several sensing elements and a finger placed on thesurface of the fingerprint sensor. Acquisition of a partial fingerprintimage may be performed using a fingerprint sensor comprising a pluralityof sensing elements arranged in a two-dimensional manner, where thisfingerprint sensor is configured to in a single image capture themajority of a person's complete fingerprint.

As an alternative, a line sensor may be used for acquiring a fingerprintimage, sometimes also referred to as a swipe sensor. The line sensor istypically a thin, rectangular shaped device configured to obtain aplurality of small overlapping images, or snapshots, as a finger isswiped past the sensor. A complete fingerprint image is obtaining byprocessing these snapshots to form a composite image. The compiling ofthe smaller images into a complete fingerprint is typically referred toas “stitching” the images.

Successful stitching for forming the complete fingerprint requiresextensive computing resources and the collection of a large plurality ofoverlapping images. Powerful microprocessors, significant amounts ofmemory, and a relatively long processing time are required to adequatelyprocess the fingerprints.

An exemplary implementation trying to improve this fallback is disclosedin US 2007/0274575. Specifically, US 2007/0274575 discloses a method andsystem for reconstructing seamless fingerprint image from set of imageslices, or frames, from a fingerprint swipe sensor. According to US2007/0274575, minutiae extraction may be performed separately over asub-portion rather than over the full or complete fingerprint.

Even though US 2007/0274575 introduces an interesting approach tofingerprint feature extraction from portions of a complete fingerprint,the disclosed approach is highly dependent on the overlap of imageslices acquired using the fingerprint sensor, resulting in a less thanrobust system. Thus, there appears to be room for further improvement inregards to efficient and robust processing of fingerprints, specificallyin relation to fingerprints collected when a user swipes his/her fingerover the fingerprint sensor.

SUMMARY OF THE INVENTION

In view of above-mentioned and other drawbacks of the prior art, it isan object of the present invention to provide an improved method forforming a fingerprint using a fingerprint sensing system. In particular,the present inventors have found that it would be desirable to determinea relation between separate image portions collected using a fingerprintsensor for improving the robustness and efficiency of the fingerprintsensing system in cases where only separated portions of the fingerprintare made available.

According to an aspect of the present invention, it is thereforeprovided a method of forming a fingerprint representation of afingerprint pattern of a finger captured using a fingerprint sensor, themethod comprising the steps of acquiring a first partial image of acorresponding first portion of a fingerprint pattern using thefingerprint sensor, extracting a first fingerprint feature from thefirst partial image, acquiring a second partial image of a correspondingsecond portion of the fingerprint pattern using the fingerprint sensor,the second partial image being non-overlapping with the first partialimage, extracting a second fingerprint feature from the second partialimage, determining a first parameter for each of the first and thesecond fingerprint feature, the first parameter being at least one of arepresentation of a relative spatial fingerprint position for therespective fingerprint feature and a representation of a relation of thefingerprint feature to the finger, and forming the fingerprintrepresentation by correlating the first parameter for the firstfingerprint feature with the first parameter for the second fingerprintfeature.

The present invention is based upon the realization that it in someinstances may be desirable to place less constrains during the formationof a fingerprint representation, such as during an enrollment process.Specifically, in some instances non-overlapping image portions will beacquired when operating the fingerprint sensor, for example due toswiping the finger over the sensor at the “wrong speed”, some errors inimage acquisition, etc. In such a scenario it may still be so that theinformation being collected may be used rather than discarding the fullfingerprint acquisition and starting over again (as is the normal priorart approach). However, to be able to make use of the acquiredinformation it is necessary to determine some form of information as toa relation between each of the acquired partial images of the finger. Itshould be understood that the disclosed concept is applicable also insituation where e.g. a non-overlapping partial image is correlated withplurality of mutually e.g. partly overlapping partial images (i.e. atleast one image is non-overlapping is with the rest of the acquiredimaged).

In one embodiment, the mentioned relative spatial fingerprint positionfor each of the partial images may be estimated by determining at leastone of a speed of the finger being swiped over the sensor or bydetermining a duration between the acquisition of the different partialimages. In another embodiment it may also be possible to correlate aspecific fingerprint feature for a partial fingerprint image withinformation (features) from a previously formed fingerprintrepresentation. As such, it may for example be possible to determine an“absolute” position of one of the first and the second fingerprintfeatures and thereby determine a relation for the other one of the firstand the second fingerprint features. If only one of the first and thesecond fingerprint features are included with the previously formedfingerprint representation, it may be possible to append the fingerprintrepresentation with the “non available” fingerprint feature.Accordingly, it may be possible to build a complete fingerprintrepresentation “over time”.

As mentioned above, the first parameter may also be a representation ofa relation of the fingerprint feature to the finger itself, typically tothe same finger of a (single user). As such, the features may notnecessarily need to be directly spatially related with each other, butinstead connected to the specific finger of the single user. This willaccordingly allow for “connecting” a plurality of fingerprint featuresto a specific finger, without necessarily having to rely on theirinterrelated spatial relation to each other.

In an embodiment, an electronic device provided with the fingerprintsensing system could be arranged in an “enrolment mode”, where a user isrequested to use the fingerprint sensor, such that at least a couple offingerprint features may be determined (e.g. including their spatialrelation). Subsequently, every time the user would touch/swipe/contactthe fingerprint sensor, for example during normal non-fingerprintsensing related use of the electronic device; the previously determinedfingerprint representation could be appended with further fingerprintfeatures. Typically, the previously determined fingerprintrepresentation is only appended in situations where the electronicdevice is certain (above a predetermined threshold of reliability) thatthe electronic device is operated by the correct user.

The present invention is also useful in situations where there samplinginterrupts are happening, for example where the fingerprint sensingsystem is comprised with an electronic device and the fingerprintsensing system is controlled by a main control processor. The maincontrol process may for example request for the acquisition of aplurality of partial fingerprint images, but could then potentially gointo an interruption mode where the main processor is not able to fullyhandle all of the received fingerprint images (for example due toanother task having a higher priority for execution).

Instead of discarding the data, i.e. the plurality of partialfingerprint images that where in fact collected, the data could still beused but with the necessity to determine some form of relation betweeneach of the received partial fingerprint images. In such a case it couldfor example be possible to “number” each of the partial fingerprintimages during their acquisition, i.e. from 1-N, and have knowledge ofthe time difference between sampling of each of the partial fingerprintimages. As such, it may for example be possible to determine anestimation of a relation between for each of the partial fingerprintimages and accordingly for each of the determined fingerprint features.

Thus, it may in accordance to the invention be possible to improve theefficiency of the fingerprint sensing system in cases where onlyseparated portions of the fingerprint are available. It should beunderstood that it may be possible, and within the scope of theinvention to combine the speed/duration between acquired partial imageswith the concept of making use of correlation with a previouslydetermined fingerprint representation.

According to another aspect of the present invention, there is provideda fingerprint sensing system, comprising a fingerprint sensor, a memory,and processing circuitry for forming a fingerprint representation of afingerprint pattern of a finger captured when swiped over thefingerprint sensor, the processing circuitry connected to thefingerprint sensor and further configured for acquiring a first partialimage of a corresponding first portion of a fingerprint pattern usingthe fingerprint sensor, extracting a first fingerprint feature from thefirst partial image, acquiring a second partial image of a correspondingsecond portion of the fingerprint pattern using the fingerprint sensor,the second partial image being non-overlapping with the first partialimage, extracting a second fingerprint feature from the second partialimage, determining a first parameter for each of the first and thesecond fingerprint feature, the first parameter being at least one of arepresentation of a relative spatial fingerprint position for therespective fingerprint feature and a representation of a relation of thefingerprint feature to the finger, and forming the fingerprintrepresentation by correlating the first parameter for the firstfingerprint feature with the first parameter for the second fingerprintfeature. This aspect of the invention provides similar advantages asdiscussed above in relation to the previous aspect of the invention.

The fingerprint sensor may, as indicated above, be implemented using anykind of currently or future fingerprint sensing principles, includingfor example capacitive, optical, or thermal sensing technology. However,at present capacitive sensing is most preferred. Both one andtwo-dimensional sensors are possible and within the scope of theinvention.

The fingerprint sensing system preferably forms part of an electronicdevice further comprising a device controller for providing saidauthentication request for said finger to the fingerprint sensingsystem, receiving said authentication signal from the fingerprintsensing system, and performing at least one action only if saidauthentication signal indicates authentication success. Such an actionmay for example be for unlocking a locked portable device, e.g. in casethe portable device is a mobile phone. The at least one action may ofcourse be any type of action suitably used when there is a need toauthenticate a finger.

According to a further aspect of the present invention, there isprovided a computer program product comprising a computer readablemedium having stored thereon computer program means for controlling aportable electronic device, the portable electronic device comprising afingerprint sensing system including a fingerprint sensor and processingcircuitry, the fingerprint sensing system adapted for forming afingerprint representation of a fingerprint pattern of a finger capturedwhen swiped over the fingerprint sensor, wherein the computer programproduct comprises code for acquiring a first partial image of acorresponding first portion of a fingerprint pattern using thefingerprint sensor, code for extracting a first fingerprint feature fromthe first partial image, code for acquiring a second partial image of acorresponding second portion of the fingerprint pattern using thefingerprint sensor, the second partial image being non-overlapping withthe first partial image, code for extracting a second fingerprintfeature from the second partial image, code for determining a firstparameter for each of the first and the second fingerprint feature, thefirst parameter being at least one of a representation of a relativespatial fingerprint position for the respective fingerprint feature anda representation of a relation of the fingerprint feature to the finger,and code for forming the fingerprint representation by correlating thefirst parameter for the first fingerprint feature with the firstparameter for the second fingerprint feature. Also this aspect of theinvention provides similar advantages as discussed above in relation tothe previous aspects of the invention.

The processing circuitry preferably includes an ASIC, a micro processoror any other type of computing device. Similarly, a software executed bythe processing circuitry for operating the inventive functionality maybe stored on a computer readable medium, being any type of memorydevice, including one of a removable nonvolatile random access memory, ahard disk drive, a floppy disk, a CD-ROM, a DVD-ROM, a USB memory, an SDmemory card, or a similar computer readable medium known in the art.

In summary, present invention generally relates to a method for forminga fingerprint using a fingerprint sensing system, and specifically tothe possibility of allowing the formation of a fingerprint when onlyseparated portions of the fingerprint is available. The invention alsorelates to a corresponding electronic device and to a computer programproduct.

Specifically, non-overlapping image portions are acquired by afingerprint sensor. A fingerprint feature is extracted for each of theimage portions. For each fingerprint feature there is determined a firstparameter, such as being a relative spatial fingerprint position (i.e.relating to the finger). In a typical use-case, the fingerprint sensor(continuously) captures fingerprint image data e.g. during averification process for verifying a finger of a user. Unfortunately, insome cases it may be so that some portions of the image data will beunavailable, for example in a situation where a host processor beingarranged to receive the image data needs to handle something else, forexample a phone call.

The typical prior-art approach is to delete all of the (partly) capturedimage data and request the user to once again place his finger at thefingerprint sensor such that a new fingerprint image may be acquired. Bymeans of the invention, the first parameter (having a specific relationto “its” fingerprint feature) is used for relating the (first andsecond) fingerprint features to each other. The invention accordinglyallows also for “partly complete” fingerprint images to be used fordetermining a fingerprint representation of a finger of the user.

Advantages with the invention include an improved robustness andefficiency of the fingerprint sensing system in cases where onlyseparated portions of the fingerprint are available. This is accordingto the invention achieved by determining and correlating a relationbetween each of the separate image portions.

Further features of, and advantages with, the present invention willbecome apparent when studying the appended claims and the followingdescription. The skilled addressee realize that different features ofthe present invention may be combined to create embodiments other thanthose described in the following, without departing from the scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of the invention, including its particular featuresand advantages, will be readily understood from the following detaileddescription and the accompanying drawings, in which:

FIG. 1 schematically exemplify an electronic device according to thepresent invention, in the form of a mobile phone comprising anintegrated fingerprint sensor;

FIG. 2 schematically shows the fingerprint sensor array comprised in theelectronic device in FIG. 1;

FIG. 3 illustrates a sequence of non-overlapping partial fingerprintimages collected using the fingerprint sensor;

FIG. 4 conceptually illustrates a fingerprint sensing system accordingto a currently preferred embodiment of the invention; and

FIG. 5 is a flowchart disclosing the exemplary steps of the inventiontypically performed in conjunction with the fingerprint sensing systemof FIG. 4.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which currently preferredembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided for thoroughness and completeness, and fully convey the scopeof the invention to the skilled person. Like reference characters referto like elements throughout.

Turning now to the drawings and to FIG. 1 in particular, there isschematically illustrated the electronic device according to the presentinvention, in the form of a mobile phone 100 with an integratedfingerprint sensor 102 and a display unit 104 with a touch screeninterface 106. In this embodiment the fingerprint sensor 102 and thedisplay unit 104 are together arranged at the front side of the mobilephone 100. The fingerprint sensor 102 may, for example, be used forunlocking the mobile phone 100 and/or for authorizing transactionscarried out using the mobile phone 100, etc. The fingerprint sensor 102may of course also be placed on the backside of the mobile phone 100.

Preferably and as is apparent for the skilled person, the mobile phone100 shown in FIG. 1 further comprises a first antenna for WLAN/Wi-Ficommunication, a second antenna for telecommunication communication, amicrophone, a speaker, and a phone control unit. Further hardwareelements are of course possibly comprised with the mobile phone. Itshould furthermore be noted that the invention may be applicable inrelation to any other type of portable electronic device, such as alaptop, a remote control, a tablet computer, or any other type ofpresent or future similarly configured device.

With reference to FIG. 2, there is conceptually illustrated a somewhatenlarged view of the fingerprint sensor 102. In the case of employing acapacitive sensing technology, the fingerprint sensor 102 is configuredto comprise a plurality of sensing elements. A single sensing element(also denoted as a pixel) is in FIG. 2 indicated by reference numeral202.

FIG. 3 conceptually exemplifies the collection of a first 302 and asecond 304 non-overlapping partial fingerprint image. In the illustratedembodiment, the first 302 and the second 304 partial fingerprint imageis separated by a “blank” section 306, for example due to a situationwhere the fingerprint sensor 102 was interrupted from collecting imagedata. As mentioned above, in a prior art situation the complete image,including sections 302, 304 and 306 would be discarded and the userwould be required to place or swipe his finger onto the fingerprintsensor 102 for capturing a new (complete) fingerprint image. However, inaccordance to the invention the first 302 and the second 304 partialfingerprint image would still be used e.g. in an enrollment processand/or for verifying/authenticating the user, as will be furtherelaborated below in relation to FIG. 5.

In accordance with the invention, the fingerprint sensor 102 and theoperation for forming the fingerprint representation is achieved using aconceptualized fingerprint sensor system 400 as is illustrated in FIG.4. The fingerprint sensor system 400 in turn, typically, forms part ofthe electronic device/mobile as is exemplified in FIG. 1.

The fingerprint sensor system 400 comprises the fingerprint sensor 102and a processing circuitry, such as a control unit 402 for controllingthe fingerprint sensor 102. The fingerprint sensor system 400 furthercomprises a memory, such as a database 406. During operation of thefingerprint sensor system 400, with further reference to FIG. 5, thefingerprint sensor 102 is operated to acquire, S1, the first partialfingerprint image 302. The first partial fingerprint image 302 is thenprovided to and processed by the control unit 402 for extracting, S2, afirst fingerprint feature, such as based on minutiae extraction.Subsequently, the second partial fingerprint image 304 is acquired, S3,by the fingerprint sensor 102 and a second fingerprint feature isextracted, S4. It should of course be understood that more than a singlefingerprint feature could be extracted from each of the first and thesecond partial fingerprint image 302, 304. Also, more than the first 302and the second 304 partial images could of course be acquired by thefingerprint sensor 102.

Once first fingerprint feature has been extracted for each of the firstand the second partial fingerprint image 302, 304, a first parameter isdetermined, S5, for each of the respective first fingerprint features.The first parameter is, as discussed above, at least one of arepresentation of a relative spatial fingerprint position for therespective fingerprint feature and a representation of a relation of thefingerprint feature to the finger.

In a possible embodiment of the invention it could be possible todetermine a time of when the first and the second partial fingerprintimage 302, 304 were acquired, such as an acquisition time stamp for eachof the first partial fingerprint image 302, 304. The acquisition timestamp could in turn be used for determining a duration betweenrespective partial fingerprint images 302, 304. Hence, by combining thedetermined duration with a known or an estimated e.g. speed of swipingthe finger over fingerprint sensor 102 (i.e. in case of a fingerprintswipe sensor), it will be possible to estimate a relative spatialfingerprint position for the respective fingerprint features extractedfrom the respective partial fingerprint images 302, 304 and thus anessentially absolute spatial relation between the fingerprint featuresof relating to the respective partial fingerprint images 302, 304.

Accordingly, even though the partial fingerprint images 302, 304 arenon-overlapping and it would be impossible to stitch the partialfingerprint images 302, 304 with each other, the inherent informationpresent in relation to each of the partial fingerprint images 302, 304could still be used for forming, S6, a fingerprint representation.

In a possible embodiment of the invention, the fingerprintrepresentation for a specific user could also be formed based on a knownrelation between a specific finger of the user and extracted features.That is, it could alternatively, or also, be possible to determine thefirst parameter for each of the first and the second fingerprint featurebased on a representation of a relation of the fingerprint feature tothe specific finger. In such a scenario it will be necessary to becertain, at least above a predetermined threshold, that a specific userin fact is handling and operating the fingerprint sensing system 400comprising the fingerprint sensor 102.

In one exemplary embodiment, the identity for the user may be in someinstances be pre-allocated to a specific device comprising thefingerprint sensing system 400. One example of such a scenario is whenthe fingerprint sensor system 400 is comprised with the mobile phone100. In such a case, the mobile phone 100 is typically personal, i.e.only used by a single user. This type of implementation may of course bepossible in relation to other type of devices, e.g. a laptop, table,etc., or when the fingerprint sensor system 400 is used with a singleuser desktop computer.

Accordingly, when for example using the disclosed method for enrolling auser, a secondary identification method such as by means of a PIN codecould be used for achieving a certainty for the user to be above thepredetermined threshold. In such an embodiment it could be possible toacquire partial fingerprint images “in the background”, for example in acase where the fingerprint sensor 102 is arrange “behind” a home buttonof the mobile phone 100. As soon as the identity of the user has beendetermined, the fingerprint sensor 102 could be configured to acquirefingerprint images “as often as possible”. Although the acquired partialfingerprint images are not necessarily completely spatially related toeach other, the fingerprint features extracted are sure to relate to thesame user. It could thus be possible to store, within the database 404,the extracted fingerprint features over time, where the first parameterfor each of the stored fingerprint features would be knowledge of arelation to the specific user.

The fingerprint representation would thus be allowed to accumulate overtime, e.g. starting from including only a few correlated fingerprintfeatures. Once an adequate/predetermined number of fingerprint featureshave been stored in the database 404, it could be possible to use these(over time collected and stored) fingerprint features in also in averification process. Hence, the user has in such a case been“background enrolled” with the fingerprint sensing system 400 withouthaving to go through the sometimes time consuming enrollment process.Thus, after some time the fingerprint sensing system 400 may inform theuser of the possibility of using the fingerprint sensing system 400 alsofor verification, purchases, etc.

Such a verification process, typically also performed by the controlunit 402, would again compare fingerprint features extracted during theverification process with the enrolled fingerprint features stored inthe database 404. The control unit 402 would as a result output a signalindicative of a result of the comparison. In case of a positivecomparison, the signal may be used for unlocking the mobile phone 100,for authenticating the user with the mail account, and/or bankingapp/client. In case of a negative comparison, the user may once again berequested to go through the process of fingerprintverification/authentication.

The control functionality of the present disclosure may be implementedusing existing computer processors, or by a special purpose computerprocessor for an appropriate system, incorporated for this or anotherpurpose, or by a hardwired system. Embodiments within the scope of thepresent disclosure include program products comprising machine-readablemedia for carrying or having machine-executable instructions or datastructures stored thereon. Such machine-readable media can be anyavailable media that can be accessed by a general purpose or specialpurpose computer or other machine with a processor. By way of example,such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROMor other optical disk storage, magnetic disk storage or other magneticstorage devices, or any other medium which can be used to carry or storedesired program code in the form of machine-executable instructions ordata structures and which can be accessed by a general purpose orspecial purpose computer or other machine with a processor. Wheninformation is transferred or provided over a network or anothercommunications connection (either hardwired, wireless, or a combinationof hardwired or wireless) to a machine, the machine properly views theconnection as a machine-readable medium. Thus, any such connection isproperly termed a machine-readable medium. Combinations of the above arealso included within the scope of machine-readable media.Machine-executable instructions include, for example, instructions anddata which cause a general purpose computer, special purpose computer,or special purpose processing machines to perform a certain function orgroup of functions.

Although the figures may show a sequence the order of the steps maydiffer from what is depicted. Also two or more steps may be performedconcurrently or with partial concurrence. Such variation will depend onthe software and hardware systems chosen and on designer choice. Allsuch variations are within the scope of the disclosure. Likewise,software implementations could be accomplished with standard programmingtechniques with rule based logic and other logic to accomplish thevarious connection steps, processing steps, comparison steps anddecision steps. Additionally, even though the invention has beendescribed with reference to specific exemplifying embodiments thereof,many different alterations, modifications and the like will becomeapparent for those skilled in the art.

In addition, variations to the disclosed embodiments can be understoodand effected by the skilled addressee in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. Furthermore, in the claims, the word “comprising” doesnot exclude other elements or steps, and the indefinite article “a” or“an” does not exclude a plurality.

What is claimed is:
 1. A method of forming a fingerprint representationof a fingerprint pattern of a finger captured using a fingerprintsensor, the method comprising the steps of: acquiring a first partialimage of a corresponding first portion of a fingerprint pattern usingthe fingerprint sensor; extracting a first fingerprint feature from thefirst partial image; acquiring a second partial image of a correspondingsecond portion of the fingerprint pattern using the fingerprint sensor,the second partial image being non-overlapping with the first partialimage; extracting a second fingerprint feature from the second partialimage; determining a first parameter for each of the first and thesecond fingerprint feature, the first parameter being at least one of arepresentation of a relative spatial fingerprint position for therespective fingerprint feature and a representation of a relation of thefingerprint feature to the finger, wherein determining the firstparameter for each of the first and the second fingerprint featurecomprises correlating at least one of the first and the secondfingerprint features with a fingerprint feature of a previously acquiredfingerprint representation; and forming the fingerprint representationby correlating the first parameter for the first fingerprint featurewith the first parameter for the second fingerprint feature.
 2. Themethod according to claim 1, wherein the second partial image isacquired at a subsequent instance in time in comparison to theacquisition of the first partial image, and the step of determining thefirst parameter for each of the first and the second fingerprint featurecomprises at least one of: determining a duration between theacquisition of the first and the second partial image, and determining aspeed of the finger being swiped over the fingerprint sensor.
 3. Themethod according to claim 2, further comprises the step of determining aspatial relation between the first and the second fingerprint featurebased on the first parameter.
 4. The method according to claim 1,wherein the first parameter for the first fingerprint feature and thefirst parameter for the second fingerprint feature are related to thesame finger of a user.
 5. The method according to claim 1, wherein thestep of forming the fingerprint representation comprises appending thepreviously acquired fingerprint representation with at the least one ofthe first and the second fingerprint feature.
 6. The method according toclaim 1, wherein the step of correlating at least one of the first andthe second fingerprint features with a fingerprint features of apreviously acquired fingerprint representation is part of a process ofenrolling the finger.
 7. The method according to claim 1, wherein theextraction of the first fingerprint feature from the first partial imageis performed independent on the extraction of the second fingerprintfeature from the second partial image.
 8. The method according to claim1, wherein the fingerprint sensor is a line sensor.
 9. The methodaccording to claim 2, further comprising the step of determining theposition of the finger at an instance in time of acquiring the first andthe second partial image.
 10. The method according to claim 9, whereinthe fingerprint sensor is arranged adjacently to a touch display, andthe step of determining the position of the finger is based on datareceived from the touch display.
 11. The method according to claim 1,further comprising the steps of acquiring a fingerprint templaterepresentative of an enrolled fingerprint of a finger, andauthenticating the formed fingerprint representation.
 12. A fingerprintsensing system, comprising: a fingerprint sensor; a memory; andprocessing circuitry for forming a fingerprint representation of afingerprint pattern of a finger captured using the fingerprint sensor,the processing circuitry connected to the fingerprint sensor and furtherconfigured for: acquiring a first partial image of a corresponding firstportion of a fingerprint pattern using the fingerprint sensor;extracting a first fingerprint feature from the first partial image;acquiring a second partial image of a corresponding second portion ofthe fingerprint pattern using the fingerprint sensor, the second partialimage being non-overlapping with the first partial image; extracting asecond fingerprint feature from the second partial image; determining afirst parameter for each of the first and the second fingerprintfeature, the first parameter being at least one of a representation of arelative spatial fingerprint position for the respective fingerprintfeature and a representation of a relation of the fingerprint feature tothe finger, wherein determining the first parameter for each of thefirst and the second fingerprint feature comprises correlating at leastone of the first and the second fingerprint features with a fingerprintfeature of a previously acquired fingerprint representation; and formingthe fingerprint representation by correlating the first parameter forthe first fingerprint feature with the first parameter for the secondfingerprint feature.
 13. The fingerprint sensing system according toclaim 12, wherein said fingerprint sensor is a capacitive fingerprintsensor.
 14. The fingerprint sensing system according to claim 12,wherein the fingerprint sensor is a line sensor.
 15. The fingerprintsensing system according to claim 12, wherein the processing circuitryis further configured for: acquiring a fingerprint templaterepresentative of an enrolled fingerprint of a finger, andauthenticating the formed fingerprint representation.
 16. An electronicdevice, comprising: the fingerprint sensing system according to claim12; and at least one processor for: providing an authentication requestfor said finger to the fingerprint sensing system; receiving anauthentication signal from the fingerprint sensing system; andperforming at least one action only if said authentication signalindicates authentication success.
 17. The electronic device according toclaim 16, wherein the electronic device is a mobile phone.
 18. Acomputer program product comprising a non-transitory computer readablemedium having stored thereon computer program means for controlling aportable electronic device, the portable electronic device comprising afingerprint sensing system including a fingerprint sensor and processingcircuitry, the fingerprint sensing system adapted for forming afingerprint representation of a fingerprint pattern of a finger capturedwhen swiped over the fingerprint sensor, wherein the computer programproduct comprises: code for acquiring a first partial image of acorresponding first portion of a fingerprint pattern using thefingerprint sensor; code for extracting a first fingerprint feature fromthe first partial image; code for acquiring a second partial image of acorresponding second portion of the fingerprint pattern using thefingerprint sensor, the second partial image being non-overlapping withthe first partial image; code for extracting a second fingerprintfeature from the second partial image; code for determining a firstparameter for each of the first and the second fingerprint feature, thefirst parameter being at least one of a representation of a relativespatial fingerprint position for the respective fingerprint feature anda representation of a relation of the fingerprint feature to the finger,wherein determining the first parameter for each of the first and thesecond fingerprint feature comprises correlating at least one of thefirst and the second fingerprint features with a fingerprint feature ofa previously acquired fingerprint representation; and code for formingthe fingerprint representation by correlating the first parameter forthe first fingerprint feature with the first parameter for the secondfingerprint feature.